Fast-moving particle is missing when poker-faced (14)
I believe the answer is:
expressionless
'poker-faced' is the definition.
(synonyms)
'fast-moving particle is missing' is the wordplay.
I cannot quite understand how this works, but
'fast' could be 'express' (I've seen this before) and 'express' is located in the answer.
'particle' could be 'ion' (I've seen this in another clue) and 'ion' is found in the answer.
The remaining letters 'less' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'when' is the link.
Can you help me to learn more?
(Other definitions for expressionless that I've seen before include "Deadpan" , "Betraying no emotion" , "having nothing to say" , "showing no emotion" , "lost for words?" .)